Panicking Doesn't Help, We Need FactsThe crisis at the Japanese nuclear power plants continues, and if you've been paying attention to the media, things seems to be getting progressively worse. But it's hard to know exactly what is going on, as there are many slightly contradictory reports, and a lot of speculation and fuzzy language (for example, a radiation level "thousands of times higher than normal" might not be very dangerous if the normal level is very low). I'm no expert on nuclear meltdowns, but I know a little bit, just enough to compile a list of the questions that I think are most important to answer right now. Read on for the list and my tentative answers.

What is the cooling situation in the reactor cores and storage pond? Is water still being pumped in, even if at a reduced rate?

This is a very important question because the cores are cooling down by themselves over time, but until they become cool enough to not melt through the reactor vessel, water needs to be used to dissipate that heat.

The latest reports are that seawater is still being pumped, but it is a "struggle" and they are using "temporary fire pumps", so that's definitely not optimal.

What kind of radioactive materials/isotopes have been released so far? Where? In what quantities?

That's another important question, because there are different types of radiation (some are stopped by a piece of paper or surface skin, other types can go through thicker materials) and different types of radioactive isotopes; some have a very short half-life, which means that they decay very rapidly into lighter atoms, while others take a lot longer to decay.

There's a trade off . Radioactive isotopes with a very short half-life are usually more radioactive, but they become safe much faster, while long-lived radioactive isotopes can stay radioactive for a lot longer, but they are less dangerous.

on 15 March a dose rate of 11.9 millisieverts (mSv) per hour was observed. Six hours later, at 06:00 UTC on 15 March a dose rate of 0.6 millisieverts (mSv) per hour was observed.

These observations indicate that the level of radioactivity has been decreasing at the site.

As reported earlier, a 400 millisieverts (mSv) per hour radiation dose observed at Fukushima Daiichi occurred between units 3 and 4. This is a high dose-level value, but it is a local value at a single location and at a certain point in time.

So while it isn't yet clear what isotopes have been released, they probably came out when steam was vented to reduce pressure inside the containment vessel, and the dosage appears to be relatively low so far (10 mSv is the equivalent of a CT scan) and you would have to be right there to get these dosages (the power plant's control room is heavily shielded from radiation).

What kind of radiation do they emit?

Once again, I'm no expert. But I do know that there are many kinds of ionizing radiation, and it would be important to know what type the people near the nuclear power plants are at risk of being exposed to. As you can see in the image above, alpha radiation is easily stopped and does go very far ("In general, external alpha radiation is not harmful since alpha particles are effectively shielded by a few centimeters of air, a piece of paper, or the thin layer of dead skin cells. Even touching an alpha source is usually not harmful"), but gamma rays are harder to avoid ("Gamma rays and neutrons are more penetrating, causing diffuse damage throughout the body (e.g. radiation sickness), increasing incidence of cancer rather than burns."). See Matt's post for more on this.

If the reactor vessel melts down, how likely is it that the molten core could get through the containment building's floor? And if it did, what would happen?

If the molten core stays hot enough for long enough to melt through the reactor vessel (which has 6.7-inch-thick steel walls and 8.4-inch-thick steel for its roof and floor) and into the containment building, what then? My understanding is that the radioactive fuel and fission byproducts would still be shielded from the outside, but would this make it much harder to keep cooling them down and to vent steam to reduce pressure? Would it increase the radiation that could escape through vented steam and gases?

How fast are the fission byproducts cooling down?

That's another important variable. How long until the cores are cool enough that they can't melt through the containment layers anymore? Days? Weeks?

Are the explosions that we've seen so far all been caused by venting to release pressure, or are some of those uncontrolled explosions?

At high temperatures, water can split into hydrogen and oxygen, which can cause explosions, especially when the high pressure gases and steam are vented to the outside. Are the explosions we've seen so far all of that type (like the one at Fukushima Daiichi 2 this morning)?